Magisk 25+ on a custom ROM: ramdisk + vbmeta, but no boot.img

According to the release notes in [Magisk v25 there was significant rewrite](https://topjohnwu.github.io/Magisk/changes.html) . The installation instructions advocated now say flashing Magisk in recovery is > This installation method is **deprecated and is maintained with minimum effort.** YOU HAVE BEEN WARNED! Instead they're pushing a new ["Patching Image"](https://topjohnwu.github.io/Magisk/install.html#patching-images) method. My device has a vbmeta and a ramdisk, but I don't understand what they mean with boot.img. What is that? I'm flashing LineageOS with Micro G, which has * lineage-18.1-20220703-microG-instantnoodlep.zip There is no boot.img there, and even in the zip there is no .img. So how do you get the patched method working with a custom ROM that isn't an .img? All the instructions say is > If your device has boot ramdisk, get a copy of the boot.img. But it doesn't say where to get the boot.img?